The long-term objectives of the research are to define the morphological versus immunocytochemical characteristics of the differentiating efferent system in the mouse at the light and electron microscope levels. The isolated organ of Corti in culture will be used as an ultimate control in discerning the innervation provided locally by the spiral ganglion cells from that of central origin. Of special importance will be the study of efferent-like vesiculated endings and their ribbon-afferent/efferent reciprocal synapses with the inner hair cells in the organ of Corti in culture and in the intact animal to determine their local source and immunocytochemical character. The immediate goal is to identify the diversity within the efferent innervation and to determine the developmental sequence of the different subunits. Specifically, we propose: l) to study the detailed developmental pattern of neurofilament immunoreactivity (which labels all neuronal elements) in the intact animal and in culture as a guide for interpreting individual immunocytochemical patterns of cochlear innervation; 2) to study CGRP expression in the innervation of the hair cells and its subsequent extinction from the outer hair cell region; 3) to study immunocytochemically at the electron microscope level the neuroactive substances AChE, ChAT, GAD/GABA, CGRP and enkephalins to determine the sequence of their differentiation and presumptive colocalization; 4) to study the ultrastructural identity of the distinct, new pathway in the inner spiral bundle which retains GAP-43 (growth-associated protein-43) activity in the adult mouse. The most important aspects of the proposed study are first, to discern the neurosensory efferent specializations associated with the inner hair cells presumed to be of cochlear origin; second, to define the ultrastructure and developmental sequence of efferent innervation, and lastly to distinguish between the transitory developmental patterns and their counterparts in the maturing animal. We hope that these studies will transcend species-lines and help us to better understand our own auditory system.